(1) Field of the Invention
The present invention relates to an optical imaging device having a short focal length and transmitting an strip image, and particularly, to an optical imaging device applicable to a facsimile, an electrophotographic copier, a LED printer and so forth.
(2) Description of the Related Art
The ongoing widespread use of facsimiles, electrophotographic copiers and LED printers has led to a demand for miniaturizing of these machines, but in such machines, since a total image is produced by scanning, an optical imaging device can transmit only a strip area of the image, at an image magnification of 1 in most cases, and therefore, a composite optical imaging device, which is an array of many small optical projecting devices arranged in the direction of the image strip is used. Each of the small optical imaging devices produces a small erect equal-magnification image on the image plane, and each small image partially overlaps the adjacent small images. Therefore, since a composite optical imaging device forms a total image by overlapping respective small images, each small optical imaging device must produce an erect equal-magnification image in the direction in which it is arranged. Usually these small optical imaging devices have small optical paths, and therefore, the optical path of the composite optical device from the object plane to the image plane is also short. The use of such an optical device enables a miniaturization of the above-mentioned machines, and furthermore, in general, such an optical device transmits a brighter image than those produced by conventional optical devices.
The known types of those optical devices are;
(a) an array of graded index lenses;
(b) an array having a plurality of small optical imaging devices each comprising a plurality of convex lenses, as disclosed, for example, in U.S. Pat. No. 3,584,952; and,
(c) an array having a plurality of optical imaging devices each comprising convex lenses and a rectangular prism, as disclosed, for example, in Japanese Unexamined Patent Publication No. 56-117,201.
The optical imaging device of the above-mentioned (a) type comprises rod lenses having a cylindrical configuration. The refractive index distribution of each of these rod lenses changes sequentially in the radial direction from the center to the periphery thereof, and it is difficult to form such a refractive index distribution and to precisely control the distribution thereof. Further, it is particularly difficult to produce a wide diameter rod lens. Therefore, when it is necessary to widen the distance between the lens array surface and the object plane, the effective F number of this device is greatly reduced and accordingly, the brightness of the obtained image is lowered.
Further, the refractive index of the medium of rod lens is changed according to the wavelength thereof, and therefore, an image produced by this optical imaging device has a large chromatic aberration.
In the optical imaging device of above-mentioned (b) type, since one convex lens produces an inverted image, each small optical imaging device produces an inverted reduction image within itself for the first time, and then converts it to an erect equal-magnification image. The optical path in each small optical imaging device passes many times through the surfaces of the lenses, and therefore, the chromatic aberration of the obtained image becomes large due to the accumulation of refractive angles at those surfaces.
Further, it is difficult to assemble this type optical imaging device because many lenses must be precisely arranged. Therefore, to reduce these production difficulties, the lenses of the respective small optical imaging devices are preferably produced as one lot for each lens layer. Nevertheless, when producing a long size array by this method, it is not easy to match each optical axis of respective layers, due to an accumulation of dimensional errors.
The optical imaging device of above-mentioned (c) type has a small chromatic aberration because the accumulation of refractive angles is kept small by using a rectangular prism for inverting the image. Nevertheless, the center of the prism and the axis of the lens must be matched for each combination. If this matching is not precise, the irregularities of the light distribution are increased.
Further, optical imaging devices of all of the above-mentioned types have disadvantages in that the obtained images are affected by periodical brightness irregularities due to the overlapping of images having light differentials between the optical axes and peripheries thereof.